Heat and turpentine recovery from pulp digesters



mm. Nm. Mm Nh hb 0N Q5 Jan'. 27, 1970 c. RosENBLAD 3,492,193

HEAT AND TURPENTINE RECOVERY FROM PULP DIGESTERS Filed Feb. 18, 1955 3 Sheets-Sheet 1 INVENTOR. Cw? EBY/QosfA/e A 0 'TO/QNEX Jan. 27, 1970 c. F. RosENBLAD 3,492,198

HEAT AND TURPENTINE RECOVERY FROM PULP DIGESTERS Filed Feb. 18, 1965 3 Sheets-Sheet 2 PRESSURE A/A/ D/ofsE/P H ow RATE PS/G 120 /PRESSURE- PS/G INVENTOR.

CUR? E ROSENBLAD ATTORNEY.

3 Sheets-Sheet 5 ATTORNEY Jaim. '27, '1970 c. F. RosENBLAD HEAT AND TURPENTINE RECOVERY FROM PULP DIGESTERS Filed Feb. 1a. 1965 United States Patent O U.S. Cl. 162-15 5 Claims ABSTRACT F THE DISCLOSURE The relief gases and blow steam from the cooking of pulp in digesters on a batch basis are led into the blow tank where they are mixed. From the blow tank the mixture is led into a direct contact condenser where the main part is condensed by direct contact with cooling liquid preferably from an accumulator containing condensate from previously condensed mixture. The uncondensed remainder of the mixture continues on to an indirect condenser or gas cooler where the condensibles, including turpentine, are condensed out. The system is kept closed and the flow to the gas cooler may be used to actuate various controls.

This application is a continuation-in-part of application Ser. No. 400,168, tiled Sept. 29, l964, now abandoned.

In digesting sulphate pulp in a batch digester in accordance with conventional practice, the nal temperature of the digester is usually about 170 C. and the pressure is about 7 atmospheres. Most of the turpentine is released during the rst period of the cook. At the end of the digestion process, the pressure is reduced by gas relief from the top of the digester to a pressure of about 4 or 5 atmospheres, after which the contents of the digester are blown into a blow tank. A smaller part of the recovery of turpentine is also released by the gas relief from full pressure down to the pressure when blowing begins. For this purpose the system is provided with means connected to the top of the digester which receives the relief gasses therefrom, removes pulp and black liquor from the gases, and condenses the gases. A turpentine recovery system of such type is shown, for example, in the patent to Nyl quist, No. 2,996,423. With such systems, the rate of recovery of turpentine is much lower than it should be on a theoretical basis, and the turpentine recovered usually requires further purication, since it is contaminated to some extent by at least small amounts of pulp and black liquor.

The above Nyquist Patent No. 2,996,423 indicates that if at least the latter part of the relief gases and vapors are taken through the blow line, either directly to a diffuser or to said diffuser after passing a central liquor trap, a considerable advantage is obtained as all the air in the system is driven out before the blow steam is introduced into the jet condenser, by which measure the condenser will function properly from the very beginning of the blow and the disadvantage of having the rst portion of pulp deposited in the diifuser under a certain overpressure and to some degree mixed with air is eliminated. If this happens, the pulp so deposited, during the later phases of the blow, when the back pressure has vanished, will boil up causing pulp and liquor to be carried over to the jet condenser; this represents a substantial loss of pulp and liquor and also severally disturbs the blow condenser system. The method as described by Nyquist may be used as well for plants using blow tanks yinstead of dilfusers, where the relief can take place directly into the blow Mice tank and not necessarily by the blow line. However, Nyquist clearly indicates that as long as the gasses and vapors are supposed to contain turpentine vapors they should be conducted to a separate turpentine recovery system of conventional type. Nyquist recommends that only after the turpentine vapors have been conventionally recovered, the relief gases and vapors be conducted to the blow line and further to the jet condenser. In accordance with the method of Nyquist, therefore, turpentine vapors prevalent in that part of the relief, wrich is conducted to the blow line, are irretrievably lost.

In accordance with the present invention, a much higher yield of turpentine is secured than was possible with prior systems, and the turpentine recovered is markedly purer. There is also less contamination of the condensate in the accumulator. These results are accomplished, generally speaking, by providing a gas cooler in series with the main or jet condenser of the system, by providing a system including the digesters, the relief gas lines connected to the digesters, the main or jet condenser, the accumulator, and the gas cooler which is closed at all times. Such system is so controlled and operated that all or substantially all of the relief gases and relief vapors, as well as the blow from the digesters, are discharged into the blow tank. From the blow tank such bases and vapors are taken through the jet or direct contact condenser, where the main part of the blow steam is condensed. Finally, the remaining gases and vapors are conducted through the gas or indirect cooler. In such cooler the gases are cooled and the remaining vapors are condensed, the condensate from such cooler being primarily a mixture of turpentine and water vapor condensate.

Preferably, in accordance with the method of the invention the ow of coolinq liquid to the direct contact condenser is so controlled in relation to the flow of blow steam therethrough that some of the blow steam passes as such, that is, in condensed form, through the direct contact condenser. As a result, the mixture of cooling liquid and condensate discharged from such condenser is maintained at or close to the boiling point. In apparatus provided for the practice of the method of the invention there is preferably provided a valve which automatically controls the ilow of cooling liquid into the direct contact condenser, and pressure responsive means disposed in the closed system either before or after the direct contact condenser for controlling such valve.

Thus, in accordance with the method of the inventlon, some of the blow steam is allowed to pass uncondensed through the jet condenser, whereby coolant passing from the jet condenser mixed with blow steam condensate will be kept at or even slightly above the boiling point and whereby the uncondensed portion of the blow steam will be condensed in the turpentine condenser, which iS connected with the jet condenser in series on the steam side and in which remaining heat will be recovered as Well as turpentine contained in said steam.

It has been assumed that turpentine would be absorbed by the coolant in the heat accumulator when the vapors pass above the liquid surface in its upper part. This is, however, not the case. The turpentine possibly absorbed by the collant during a cooler phase will, during each blow, be boiled out from the coolant when this is leaving the jet condenser at or above boiling temperature.

It has been proven that it is easy to regulate the amount of coolant to the jet condenser in relation to the blowsteam introduced to said condenser in such a way that the main part of the blow steam will be condensed leaving a small portion to pass through the condenser uncondensed by means of an automatically controlled valve in the coolant line, the movements of which are imitated by the over-pressure in the line to the surface condenser.

This over-pressure can be held at some four to twelve inches of water column up to p.s.i.g. at the beginning and prevalence of the blow to be dissipated down to close to atmospheric pressure at the end of the blow, an overpressure completely without harmful effects for the smooth completion of the blow. In some instances it may be advantageous to apply the pressure impulse in the line before the jet condenser and in some instances it may be favorable to augment the resistance in the line to the surface condenser by inserting an orifice or similar obstruction. It will also be possible to use the pressure difference on the two sides of such an orifice as pressure impulse. Earlier ways to regulate the amount of coolant flow to the jet condenser have used the temperature of the coolant leaving the jet condenser as an impulse often in combination with pressure impulses in thesystem, and these impulses have been made to actuate one or more valves in different combinations.

It is accordingly the general object of the invention to improve upon the effectiveness of blow steam recovery systems in sulphate pulp cooking.

A further object of the invention is to provide an improved method of blow steam recovery which is characterized by the improvement in the amount of turpentine recovered from the system and the purity of the thus recovered turpentine.

Another object of the invention is to provide an improved method of blow steam recovery in systems of the type described wherein the condensate in the upper layers of the accumulator is maintained at or substantially at the boiling point and wherein there is less contamination of the condensate in the accumulator.

Still another object of the invention is to provide an improved blow steam recovery system for carrying out the method of the invention.

The above and further objects and novel features of the invention will more fully appear from the following description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only, and are not intended as a definition of the limits of the invention.

In the drawings, wherein like reference characters refer to like parts throughout the several views,

FIG. 1 is a diagrammaic illustration of a rst embodiment of a heat recovery system in accordance with the invention and for carrying out the method thereof;

FIG. 2 is a composite graph of a single blow of a digester discharge cycle, said figure showing the regular flow of steam in pounds per minute and the blow pressure, both such values being plotted against blowing time in minutes; and

FIG. 3 is a diagrammatic illustration of a second embodiment of a heat recovery system in accordance with the invention and for carrying out the method thereof.

As is apparent from the above, there are illustrated herein two embodiments of heat recovery system in accordance with the invention. Both systems operate in generally the same manner throughout the predominant portions of their cycles, the system of FIG. l differing from that of FIG. 3 in that in the latter portion of the discharge cycle of the system of FIG. l the blow steam is caused to by-pass the jet or direct contact condenser and to pass directly to the gas cooler or indirect condenser. For reasons of simplicity and economy, the system of FIG. 3 is ordinarily preferred over that of FIG. l.

Considering now the features of FIG. 1 in detail, a single digester for the cooking of pulp is illustrated at 1, said digester being discharged through a suitable header to a common single blow tank, as shown here at 2. Thus, though in the description to follow the digester 1 will be referred to, it is, of course, to be understood when not otherwise specified that the digester 1 could be any one in the bank, whether the number employed is four or otherwise.

At its upper end the digester 1 has an opening neck 5 suitably closed by a cover 6. A gas relief pipe 7 extends from one side of the digester neck 5 and is in communication with the interior of the digester so that gases collecting in the upper portion thereof can be relieved through that pipe. A relief valve 8 in the pipe 7 controls the ow through that pipe and from the valve 8 the pipe continues into communication with the common gas relief header 9 for all of the digesters in the bank. This header normally continues past a normally open valve (not shown) into communication with the interior of the blow tank 2 adjacent its top 12. The top 12 of the blow tank is provided with a safety outlet pipe 16 provided with a safety pressure release damper 17.

An outlet or blow pipe 20 is connected to the top 12 of blow tank 2 as shown at 21, such pipe 20, in the illustrative embodiment, extending directly into the top, vaporcontaining space 33 of an accumulator tank 23, to be described. Mounted upon the top of the accmulator tank 23 and directly communicating with the vapor-containing space 33 in the tank is a direct condenser 22. The direct condenser 22 may be of various types, but as here illustrated, and as explained in the foregoing, it is generally of the type detailed in my co-pending application Ser. No. 220,386, above referred to. Accordingly, there is provision for cooling liquid being introduced into the top of condenser 22 at 24 through the pipe 25, and as it flows down it is distributed by bafe and direction changing means, as shown at 27, 28, 29, 30, and 31. These may be of varying numbers and types, and those here shown are merely for illustrative purposes. What is important, however, is that the main direct condenser 22 has suicient capacity for condensing blow steam introduced into it through the pipe 20 which enters the vapor containing space 33 in the accumulator at the location 89.

The accumulator tank 23 has a rounded upper end 32 which usually provides the gas chamber or vaporcontaining space 33 therein. Below that the tank is cylindrical, as shown at 34. The accumulator tank 23 provides the reservoir for hot and cooled liquid, hot in the upper portion of the tank and cooled in the lower portion achieved by a re-circulating system, to be described.

That re-circulating system for the contents of the accumulator tank consists of a withdrawing element 35 within the tank close to the upper portion of the cylindrical wayy thereof, element 35 being connected to the pipe 36 through which hot condensate from the top of the tank 23 is drawn off. Outwardly of the tank 23 the pipe 36 is connected to a discharge pump 39 which forwards hot condensate under pressure through a pipe 40. Such pipe 40 may, if desired, be connected to an indirect heat exchanger (not shown) for the heating of process water, for example.

It is desirable to maintain the top level of liquid in the accumulator tank 23 at approximately the position indicated by the line 48 therein. This is accomplished, in the illustrative embodiment, by the provision of a discharge valve 72 operated by liquid level responsive means 73, to be described.

The heat recovery system illustrated herein includes an after-condenser or indirect heat exchanger 56 which has sufficient capacity to take care of all the blow steam vapors, and non-condensable gases which are discharged through blow pipe 20 during the second portion of the discharge cycle. The turpentine formed from the condensing of its gases in such mixture runs off through the pipe 57 from the after-condenser while the remaining non-condensable portion of such gas mixture can be vented to atmosphere or otherwise disposed of through a vent pipe 58. The after-condenser 56 is provided with an ample supply of cooling water through a pipe connected to port 59 of the after-condenser. Cooling water which is discharged from the after-condenser 56 flows therefrom through a pipe 47 which may either enter the bottom of the accumulator tank 23, as shown, or lead to a discharge sump.

As above indicated, the after-condenser 56, in the first portion of the blow or discharge cycle of the vessel or digester 1, is fed with vapors and non-condensable gases from the vapor-containing space of the accumulator and/or t-he direct condenser 22, being then disconnected from direct connection to the blow pipe 20, When, however, the pressure of the blow steam drops to said predetermined Value, and the second portion of the discharge cycle begins, the after-condenser 56 is shut off from direct communication with the vapor-containing space of the accumulator 23 and/or the direct condenser 22 and is then fed with the remaining blow steam, vapors, and non-condensable gases directly from the blow pipe 20. In the illustrative system, these operations are effected automatically by the following mechanisms.

The end of the blow pipe 20, inwardly of the accumulator tank 23 at the position 89 at which the pipe enters the tank, is provided with a check valve 90 which remains open so long as the pressure of the blow steam in pipe exceeds said desired predetermined value and closes automatically when such pressure drops below such value. A second, by-pass pipe or conduit 55 is connected between conduit 20 outwardly of valve 90 and the inlet port of the after-condenser 56. A third conduit 62 is connected between conduit 55 and the direct condenser 22. It is to be understood, however, that conduit 62 may, if desired, be connected to the upper end 32 of the accumulator tank 23. Interposed in conduit 55 between conduit 20 and conduit 62 is a selectively operated shut-olf valve 60. Interposed in conduit 62 is a Valve 61 which may be selectively operated to place it in closed or open position. In said open position the valve 61 operates as a pressure-responsive relief valve to permit the passage of gases therethrough from the direct condenser 22 to the conduit 55 when such gases exceed the predetermined pressure relief value for which the valve 61 is set. Valve 61 may be set, for example, so that when it is in open position, it passes gases from condenser 22 at a pressure of 5 p.s.i.g. and above. The accumulator tank 23 is also provided with a pressure relief conduit 70 rising from the vapor-containing space thereof, pipe 70 being provided with a pressure relief valve for damper 71. Under the assumed condition of the maintenance of a pressure 5 p.s.i.g. within the vapor-containing space 33 of the accumulator during both the first and second portions of the discharge or blow of the vessel, the pressure relief valve 71 may be set to open, for example, at a pressure of 7 p.s.i.g.

In the illustrative system the direct condenser 22 is provided with cooling liquid in the manner illustrated and claimed in the application of Axel E. Rosenblad, Ser. No. 374,208, filed June 10, 1964. In accordance with such arrangement, the pipe or conduit leading to the direct condenser 22 is fed from an intake device 84 lying adjacent the bottom of the accumulator tank 23, device 84 thus being positioned to withdraw the coolest condensate from the accumulator. Condensate entering the device 84 flows through a pipe 83 to the intake port of a force pump 82 of large capacity. Connected in shunt with the pump 82 and with the pipe 25 and the lower end of the space within the accumulator tank 23 is a pipe 85 which has a selectively operable shut-off valve 86 interposed therein. The intake port 24 of condenser 22 is positioned substantially above the pump 82. When valve 86 is fully opened, it passes a sufficient portion of the output of pump 82 to maintain the level of condensate in the pipe 25 somewhat below the upper, generally horizontal run thereof. When, however, the valve 86 is closed, the pump 82 immediately delivers its full output directly to the inlet port 24 of condenser 22- Although other mechanisms for feeding cooling fluid to the condenser 22 may be employed, the mechanism here shown is particularly advantageous because of the speed with which it responds selectively to deliver cooling fluid to the direct condenser 22 when required and to shut off the flow of such uid to such condenser.

As above noted, the level of the liquid in the accumulator tank 23 is preferably maintained substantially constant as at the line 48. The discharge valve 72, positioned adjacent the lower end of tank 23, when opened -by the float mechanism 73, discharges into a surge tank 74. Condensate from the surge tank 74 is withdrawn therefrom by a pump 77 and forwarded to a heat exchanger 75 which may be of the indirect type. Heated condensate enters heat exchanger 75 through the conduit 76 and is discharged therefrom through conduit 78. Cooled water enters the heat exchanger through a conduit 79 and is discharged as hot water through a conduit 80.

The system incorporates means whereby should the pressure within the vapor-containing space 33 of the accumulator tank fall below the desired predetermined value live steam is injected directly into such space. Such means takes the form of a steam injecting pipe 87 connected to a source of live steam (not shown), a pressure controlled shut-off valve 88 being interposed in pipe 87. Valve 88, which is of conventional design, is arranged to open and discharge live steam into space 33 whenever the pressure in such space falls below said desired predetermined value.

As will appear hereinafter, the valve 60 is under the control of the check valve 90, as by means of an electrical contact on such valve interposed in a control circuit, schematically indicated at 66, connected to valve 60. The pump 82, which introduces cooling fluid to the direct condenser 22, is also preferably automatically controlled. Thus there is provided an orifice 63 in conduit 55 between the location of connection of the conduit 62 thereto and the entrance port of the after-condenser 56. Such orifice, which detects the lrate of flow of gases therethrough and thus pressure differences on the two sides thereof, is connected by a control circuit 67 to a differential cell 68 which in turn is connected through a control circuit 69 to the operating means for the lby-pass valve 86. Valve 86 is provided with a conventional control means, not specifically shown, whereby such valve is held in open position during the end of the first portion of the discharge cycle, when valve 90 is closed and valve 60 is opened. As a result, at such time the flow of cool condensate into the direct contact condenser 22 is avoided.

The described system of FIG. l operates as follows: Before the start of a blow or discharge cycle, the whole system, including the blow tank 2, is under a slight pressure, of, for example, l0 inches of water column. When the digester valve 4 opens and the blow starts, the pressure in the digester drops very rapidly, as is shown in the pressure-time graph of FIG. 2. The blow pressure drops, for example, from to 6 p.s.i.g in 61/2 minutes. During this time, which is termed the first portion of the discharge cycle, the valve 90 in blow pipe 20 is open. Valve 61 may open partly or fully in functioning as a pressure relief to keep the pressure in the accumulator and the condenser 22 at a predetermined value, for example 6 p.s.1.g.

When the pressure in the blow tank falls to the predetermined value, for example 6 p.s.i.g., the first portion of the above cycle is considered to have ended and the second portion thereof to have begun. Then the pressure control valve 90 shuts automatically, as does the check valve 61. Simultaneously with the closing of valve 90 the valve 60 opens, thereby yby-passing the remainder of the blow steam during the second portion of the discharge cycle. Accordingly, the space 33 within the accumulator 23 and Within the direct condenser 22 is shut off from the blow tank. The blow steam, including vapors and noncondensable gases, then passes through the orifice 63 directly to the after-condenser 56. As we have seen, when this occurs the differential cells 68 opens the by-pass valve 86 of the cooling fluid delivery system, thereby stopping the delievery of cooling condensate to the condenser 22.

When the blow or discharge cycle has been finished, and the glow valve 4 at the bottom of the digester 1 has been shut, valve 60 automatically closes and valve 61 assumes its open position at which it functions as a pressure relief valve. The system is now ready for a fresh blow or discharge cycle.

To get a clear picture of actual savings affected by the system and method of the invention, we can use normal figures for a 400 tons/day kraft mill employing 40 cooks a day. Assuming a blow heat of, say, 2,400,000 B.t.u. per ton of pulp, there will be 40,000,000 B.t.u. per hour to recover. Peak load when blowing may be, for example 4000 lbs. steam per minute. Water to be heated normally is preheated in evaporators or surface condensers, say up to 100 F. Used as process water in the mill, such water is heated to 160 F. min. or 180 F. max. The latter case refers to use of such water in a bleaching plant.

In comparing a pressurized system in accordance with the invention with a conventional heat recovery system, it will be assumed that the main components of the systems are the same, and that the accumulator volume is calculated to have a capacity sufficient to take care of one blow with 50% margin. It will also lbe assumed that in -both cases the hot condensate is `cooled in heat exchangers down to 170 F., and that the cooled condensate is delivered to the bottom of the accumulator.

With the conventional, non-pressurized system, the average temperature of hot condensate at the top of the accumulator is 200 F. With a pressurized system in accordance with the invention, however, hot condensate can be produced and maintained in the upper portion of the accumulator at a temperature of 230 F. when the accumulator is held at a pressure of 6 p.s.i.g. pressure. The higher temperature of hot condensate in the accumulator makes possible the above noted marked economies in the heating surface of heat exchangers utilizing the hot condensate as a source of heat, in the accumulator volume requirement, and in the requirements for pumps, pipes, and conduits for supplying cooled condensate. The maintenance of the hot condensate continuously under substantially constant pressure in the accumulator, regardless of marked variations in the pressure of the blow steam, makes possible the continuous discharge of hot condensate of substantially constant temperature. This is accomplished with the transfer of the vapors, released during the pressure drop from 6 p.s.i.. down to atmospheric pressure, directly to the after condenser. This after condenser should have ample size for preheating fresh Water to a temperature which is satisfactory for such water to be used as process water. The after condenser should also have sufficient heating surface to sub-cool the turpentine condensate mixture. The quantity of heat collected in the accumulator will be correspondingly reduced, but the temperature of the hot condensate in the described system will have a higher than conventional temperature level, varying between 230 and 212 F.

In the system of FIG. 3 parts which are similar to those of FIG. 1 are designated by the same reference characters as those employed in FIG. 1. It will therefore suffice to point out the differences in construction between the two systems and to discuss their specific differences in manner of operation. In FIG. 3 there is shown a conduit 10, to which other digesters, not shown, which are not connected to the condenser 11, are connected. The header, here designated 9 for the digester 1 shown, is connected to the header 10 so that both discharge into the upper end of the blow tank 2 through the relief conduit 9. As before, the upper end of the blow tank is connected by a blow Iconduit 10 to the direct contact condenser-accumulator combination. In this embodiment, rather than being connected to the vapor space within the upper end of the accumulator, the discharge end of the conduit 10 is connected to the upper end of the condenser 11. It is immaterial whether the condensate flows in countercurrent to the cooling liquid, as in FIG l, or in the same direction as the cooling liquid, as in FIG. 3. The accumulator 13 of FIG. 3 is shown as being provided with an overflow pipe 49 which extends from near the bottom of the tank to an inverted U-shaped portion at the desired top of the surface of the condensate in the tank, the overow pipe "being connected outwardly of the tank to a conventional trap 50. As a result, the level of condensate within the accumulator tank is maintained su-bstantially at the line 48.

In the apparatus of FIG. 3 the vapor space within the upper end of the accumulator tank 13 is connected at all times by the pipe 55 to the gas inlet end of the gas cooler 56. Such gas cooler, which is preferably of spirally wound construction, is so made and operated that it maintains its gas-connected indirectly cooled surfaces sufficiently cool to condense turpentine thereon. Interposed in the pipe 55, through which gas is fed to the gas coolerl 56, is an orifice-providing diaphragm 63 which operates a differential pressure sensing device 52 in the control circuit 67, such circuit extending to a flow responsive control 51. The diaphragm 63 also functions to throttle the flow of vapor into the gas cooler 56 to create a desirable back pressure on the vapor If desired, diaphragm 63 may be disposed in the outlet pipe 94 from the gas cooler, where it would also function as a differential pressure sensing and vapor throttling device. From control 51 there extends a control circuit 69 to the automatically controlled valve 86. Such portion of the system of FIG. 3 operates in the same manner as that of FIG. 1. Thus upon the discharge of a blow from a digester into blow tank 2 and thus into condenser 11 the control system 63, 52, 67, 5S, 69, 86 operates to discharge cooling water into the upper end of the direct contact condenser 11, the flow responsive control 51 causing cooling water to flow in proportion to the discharge of blow steam into the condenser. Upon the cessation of discharge of blow steam into the condenser, the ow of cooling water thereto is stopped.

The gas cooler 56 is fed with cooling water through a pipe 81 which has a temperature responsive valve 91 interposed therein. Valve 91 is under the control of ternperature responsive means, not shown, whereby the temperature of the water discharged from the cooler through the pipe 92 for use, for example, in the mill, is maintained constant. Uncondensed gases discharged from the gas cooler 56 pass to a vent through a pipe 96, condensed vapor from such gases being discharged from the cooler through a pipe 93.

Condensed turpentine is discharged from the cooler 56 through a pipe 94 which leads to a sub-cooler which is generally designated 95. The sub-cooler may be either of the air or water cooled type, here being shown to be of the latter type and as being supplied with coolingr Water through a pipe 97, the cooling water being discharged through a pipe 98. The resulting cooled turpentine flows downwardly through a discharge pipe 99 and into a decanter 100 from which it may 'be withdrawn through outlets 101, 102, and 103 disposed at progressively higher levels. A vertically disposed sight glass 104 is provided on the decanter as shown. Any water which flows into the decanter settles at the lower end thereof so that it may be withdrawn through a water outlet conduit 105 provided with a manually operated discharge valve 106.

As above indicated, the gas cooler 56 is so constructed and arranged and is so operated as to condense turpentine from the vapors passing therethrough. As is well-known in the art, the principal constituent of oil of turpentine is pinene, CwHls, which boils at 156 C. The vaporcontacting surfaces of the gas cooler 56 are thus maintained cool enough to condense the condensible components in the vapor flowing into the gas cooler, that is, at a temperature somewhat below 156 C.

Although a limited number of embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing specification, it is to be especially understood that various changes, such as in the relative dimensions of the parts, materials used, and the like, as well as the suggested manner of use of the apparatus of the invention, may be made therein without departing from the spirit and scope of the invention, as will now be apparent to those skilled in the art.

What is claimed is:

1. In a method for recovering turpentine from the relief gases resulting from the cooking of pulp in cellulose digesters in a batch system wherein part of the condensibles from said relief gases are condensed by bringing them into direct contact with a cool liquid in a direct contact condenser and wherein the condensate from said direct contact condensation is collected in a accumulator tank below said direct contact condenser, the improvement which comprises, passing said relief gases from the digester into a blow tank, separating said relief gases from black liquor and fibers in said blow tank, conducting said separated relief gases from the top of said blow tank into said direct contact with said cool liquid to condense said part of said gases, thereafter-drawing olf the remaining part of the said relief gases including turpentine vapors and uncondensibles from said direct contact condenser and condensing the condensibles from said remaining part including said turpentine vapors by bringing the same into indirect contact with a cooling medium with said indirect contact and direct contact condensing steps being carried out in series, and collecting the condensate, including condensed turpentine vapors, from said indirect condensation separately from the condensate 10 from said direct condensation all while maintaining said blow tank, said indirect and direct condensers and said accumulator tank in a closed system.

2. The method as in claim 1 and maintaining the same pressure throughout said closed system.

3. The method as in claim 1 and condensing turpentine containing vapors in said indirect contact condenser and separating turpentine from said condensate.

4. The method as in claim 1 and throttling the flow of said remainder of condensible gases to create a back pressure on the gas flow.

5. The method as in claim 1 and passing said remainder of condensible gases through said direct contact condenser on the way to said indirect condensation thereof and adjusting the ow of cooling liquid for said direct condensation in relation to the flow of said remainder of condensible gases from said direct contact condenser to said indirect condenser.

References Cited UNITED STATES PATENTS 3,286,763 11/1966 Jacoby.

3,289,736 12/ 1966 Rosenblad.

1,469,958 10/ 1923 Richter 162-52 X 2,803,540 8/1957 Durant et al. 162-239 X 2,808,234 10/1957 Rosenblad 165-34 S. LEON BASHOR'E, Primary Examiner ROBERT L. LINDSAY, Assistant Examiner U.S. Cl. X.R. 

